UNITED NATIONS, Feb 25 (Reuters) - Syrians are about to replace Afghans as the world's largest refugee population, fleeing a conflict where barrel bombs leave bodies in pieces and a generation of children are physically and emotionally scarred, top U.N. officials said on Tuesday.

U.N. Secretary-General Ban Ki-moon told the U.N. General Assembly that the world body would do everything to implement a U.N. Security Council resolution - adopted on Saturday to boost humanitarian aid access - and get help to millions in need.

"Supplies are ready to go into areas that have been hard to reach, and into the towns and cities that have been under siege," Ban said. "What we need is guaranteed safe passage for humanitarian supplies along key routes."

"It is incumbent on the Syrian government and all parties to the conflict to reach these agreements," he said.

Some 9.3 million Syrians - almost half the population - need help, the United Nations said. Some 2.4 million of those people have fled the country during the three-year civil war.

"Five years ago Syria was the world's second-largest refugee hosting country. Syrians are now about to replace Afghans as the present biggest refugee population worldwide," U.N. High Commissioner for Refugees Antonio Guterres said.

"It breaks my heart to see this nation that for decades welcomed refugees from other countries, ripped apart and forced into exile itself," Guterres told the 193-member U.N. assembly.

The unanimously adopted Security Council resolution demands cross-border aid access in Syria, an end to the use of weapons such as barrel bombs in cities and towns and threatens "further steps" in cases of non-compliance.

U.N. diplomats say Syria's ally Russia is unlikely to agree to any action if the Damascus government was found to be in non-compliance. But Western envoys expressed a strong intent to push for Security Council action if the resolution is ignored.

U.N. human rights chief Navi Pillay told the General Assembly that imprecise weapons, notably barrel bombs, had reportedly killed hundreds of people in February alone.

She added between Dec. 15-28, 2013: "Barrel bombs landed on more than 12 districts, exploding on residential buildings, marketplaces, a bus station, a school and in close proximity to hospitals. This resulted in inhumane loss of life."

"As described ... by one witness after an attack on a bus station, I quote, 'bodies arrived in pieces, there were parts of bodies and parts of vehicles hanging on the power cable,'" Pillay said.

'NOTHING NEW'

Pillay said tens of thousands of people were reportedly held in official and unofficial government detention centers. She also called on foreign powers to curb supplies of arms and the inflow of foreign fighters into Syria.

"The ... influx of arms and fighters from abroad diminishes the chance of reaching a political solution," Pillay said.

A second round of peace talks in Geneva broke up on Feb. 15 with international mediator Lakhdar Brahimi lamenting a failure to advance much beyond agreement on an agenda for a third round at a later date.

Aid workers on Tuesday doubted that the U.N. Security Council resolution on aid access in Syria had enough punch to push the Damascus government to allow more humanitarian convoys carrying vital supplies into the country.

Deputy U.N. aid chief Kyung-Wha Kang told the General Assembly that some 4.3 million children in Syria are affected by the conflict, while another 1.2 million are refugees.

"Children have been killed, arrested, abducted, tortured, mutilated, sexually abused, and recruited by armed groups. They have been used as human shields. They are malnourished," she said. "Syria is in danger of losing a generation of children."

"We are in a race against time. More people are slipping out of our reach as the conflict intensifies and armed groups fragment and more battle lines are drawn," Kang said.

She said humanitarian groups needed to bring in more aid workers and expertise to deliver more help to more people.

Ban called on the Syrian government to authorize more aid workers to enter the country. "It is not credible to cite bureaucratic procedures as reasons for delay when it is the government itself that controls those procedures," he said.

Syrian U.N. Ambassador Bashar Ja'afari accused the governments of some states of not being interested in providing humanitarian assistance to Syria. He blamed Saudi Arabia, Qatar and Turkey for pouring "oil on the fire of the Syrian crisis."

"Some states have agreed to lay out large sums of money in order to support arms deals to terrorists to let them get through the Syrian borders instead of spending this money on humanitarian assistance to those who need it," he said.

He was dismissive of the General Assembly briefing on Tuesday.

"These briefings, in spite of their great significance, didn't really offer anything new. They were confined to repeating what we have heard in similar meetings at the U.N.," he said. "Humanitarian questions, on which we have been briefed today, are the heart of concerns of the government of Syria." (Reporting by Michelle Nichols; Editing by Gunna Dickson)

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